Conservative former congressman Pat Toomey made it official Wednesday, signing on for a rematch with moderate Pennsylvania Sen. Arlen Specter in the 2010 Republican Senate primary.

APPat Toomey

Starting with a talk-radio interview in Allentown and a video on his new campaign Web site, Toomey said he was running to stop the "bailouts and the spending stampede" and because Specter had consistently supported "increased government spending and a liberal agenda on social, labor, immigration and national security policies."

In 2004, Toomey shocked politicos when he came within 17,000 votes -- out of 1 million cast -- of defeating Specter, despite being outspent, 4-to-1. This time, he won't have the element of surprise; Specter, anticipating his challenge, has already aired commercials attacking Toomey's record.

The announcement came on tax filing day, as conservative activists across the country held "tea parties" to protest what they see as out-of-control government spending that will lead to unsustainable debt and higher taxes.

Toomey, 47, had been exploring a run for governor until an eruption of anger in the GOP's conservative base greeted Specter's crucial vote in February for the Democrats' $787 billion economic stimulus bill. Two recent independent polls found that a narrow majority of Republicans surveyed favored someone new in the Senate seat.

Specter's campaign said in a statement that Toomey was too far to the right to win a general election in Pennsylvania, a state dominated by centrist voters that has been trending Democratic in recent years.

After all, the statement said, Toomey is in some ways more conservative than former Republican Sen. Rick Santorum, who lost his 2006 re-election bid by 18 percentage points.

"Without Senator Specter's seat, which Mr. Toomey would certainly lose, there would no longer be 41 Republican senators to filibuster and stop the Democrats from passing card check, raising taxes, and implementing President Obama's massive spending plans," said Christopher Nicholas, Specter's campaign manager.

Specter, who is seeking his sixth term, recently voted against Obama's proposed 2010 budget and announced he would oppose the so-called card-check bill that would make it easier for unions to organize. Toomey said those were politically motivated flip-flops.

Though Specter, 79, has proved a popular vote-getter with Democrats and independents in general elections, the primary could be his biggest challenge, with a smaller and more conservative Republican electorate.